The sedimentary structure and petroleum geologic significance of the ring belt of the closed lake basin: An integrated interpretation of well and seismic data of the Kong2 Member in Cangdong Sag, Central Bohai Bay Basin, China

Cangdong is a typical oil-rich sag in the Bohai Bay Basin, China. After more than 50 years of exploration and development, the Kong2 Member (the major hydrocarbon play in the sag) still has considerable residual oil and gas resource potential. To pursue replacement areas of oil and gas exploration and development, the basic geology of the entire Kong2 Member in Cangdong Sag as a unit has been reexamined, and the findings have been used to guide the secondary exploration deployment. In this study, the characteristics of sedimentary reservoirs, source rocks, and oil and gas distribution in the Kong2 Member have been systematically studied, and a sedimentary model of the ring belt-circle layer of the closed lake basin in the Kong2 Member of the Cangdong Sag, with three segments (high, middle, and low) on the profile, three ring belts (outer, middle, and inner) on the plane, and three circle layers (outer, middle, and inner) in space has been established. The ring belt and circle layer are jointly controlled by water-body differentiation in the closed lake basin, source-material supply, depositional accommodation space, and deposition base-level cycle, and they can be in round, oval, long strip, and irregular shapes. The outer ring (circle), located near the basin margin, mainly has delta-front subfacies conventional coarse-grained medium-thick sandstone and near-source structural and stratigraphic-lithologic reservoirs; the middle ring (circle), the transitional zone from the basin margin to the central basin, is dominated by fine sandstone, siltstone, and lacustrine carbonates of front delta subfacies, and it mainly contains isolated lithologic reservoirs and unconventional tight oil; the inner ring (circle) is the high-quality hydrocarbon source-rock development zone in the center of the closed lake basin, featuring a high abundance of shale, where the dolomite and siltstone of distal gravity flow right next to source rock, and fine-grained diamictite of the source reservoir in one area rich in tight oil, whereas the high-abundance shale of frequent source-reservoir interbeds is rich in shale oil. The strategy of oil and gas exploration deployment is to look for structural, stratigraphic-lithologic reservoirs in the outer circle (outside source), lithologic reservoirs in the middle circle (near source), and retained tight oil and shale oil in the inner ring (inside source). In recent years, major discoveries have been made in oil and gas exploration in the three circle layers of the Kong2 Member in the Cangdong Sag through drilling, especially in tight-oil exploration in the inner-circle layer: two sandstone sweet-spot intervals of greater than 60 m and three dolomite sweet-spot intervals of greater than 100 m have been confirmed. The maximum daily oil production of vertical wells after fracturing is up to 50 t; several hundred square kilometers of favorable exploration area has been delineated, with an estimated oil geologic resource of 100 million tons.